System and method for reducing power consumption in an optical screen pointing device

ABSTRACT

An apparatus for controlling the position of a screen pointer for an electronic device having a display screen includes a light source for illuminating an imaging surface, thereby generating reflected images. The apparatus includes a motion transducer. A lens receives the reflected images and directs the reflected images onto the motion transducer. The motion transducer includes an electronic shutter for controlling the amount of time that light is collected for image frames. The motion transducer is configured to generate digital representations of the reflected images. The motion transducer is configured to generate movement data based on the digital representations of the reflected images. The movement data is indicative of relative motion between the imaging surface and the motion transducer. A controller coupled to the light source turns the light source on only during the time that light is being collected for an image frame.

This application is a division of Ser. No. 09/912,190 filed Jul. 24,2001.

REFERENCE TO RELATED PATENTS

This Application is related to the subject matter described in thefollowing U.S. patents: U.S. Pat. No. 5,578,813, fitled Mar. 2, 1995,issued Nov. 26, 1996, and entitled FREEHAND IMAGE SCANNING DEVICE WHICHCOMPENSATES FOR NON-LINEAR MOVEMENT; U.S. Pat. No. 5,644,139, filed Aug.14, 1996, issued Jul. 1, 1997, and entitled NAVIGATION TECHNIQUE FORDETECTING MOVEMENT OF NAVIGATION SENSORS RELATIVE TO AN OBJECT; and U.S.Pat. No. 5,786,804, filed Oct. 6, 1995, issued Jul. 28, 1998, andentitled METHOD AND SYSTEM FOR TRACKING ATTITUDE. These three patentsdescribe techniques of tracking position movement. Those techniques area component in a preferred embodiment described below. Accordingly, U.S.Pat. Nos. 5,578,813, 5,644,139, and 5,786,804 are hereby incorporatedherein by reference.

This application is also related to the subject matter descnbed in U.S.Pat. No. 6,057,540, filed Apr. 30, 1998, issued May 2, 2000, andentitled MOUSELESS OPTICAL AND POSITION TRANSLATION TYPE SCREEN POINTERCONTROL FOR A COMPUTER SYSTEM; U.S. Pat. No. 6,151,015, filed Apr. 27,1998, issued Nov. 21, 2000, and entitled PEN LIKE COMPUTER POINTINGDEVICE; and U.S. patent application Ser. No. 09/052,046, filed Mar. 30,1998, entitled SEEING EYE MOUSE FOR A COMPUTER SYSTEM. These two relatedpatents and patent application describe screen pointing devices based onthe techniques described in U.S. Pat. Nos. 5,578,813, 5,644,139, and5,786,804. Therefore, U.S. Pat. Nos. 6,057,540 and 6,151,015, and U.S.patent application Ser. No. 09/052,046, filed Mar. 30, 1998, entitledSEEING EYE MOUSE FOR A COMPUTER SYSTEM, are hereby incorporated hereinby reference.

THE FIELD OF THE INVENTION

This invention relates generally to devices for controlling a cursor ona display screen, also known as pointing devices. This invention relatesmore particularly to a system and method for reducing power consumptionin an optical pointing device.

BACKGROUND OF THE INVENTION

The use of a hand operated pointing device for use with a computer andits display has become almost universal. By far the most popular of thevarious devices is the conventional (mechanical) mouse, used inconjunction with a cooperating mouse pad. Centrally located within thebottom surface of the mouse is a hole through which a portion of theunderside of a rubber-surfaced steel ball extends. The mouse pad istypically a closed cell foam rubber pad covered with a suitable fabric.Low friction pads on the bottom surface of the mouse slide easily overthe fabric, but the rubber ball does not skid. Rather, the rubber ballrolls over the fabric as the mouse is moved. Interior to the mouse arerollers, or wheels, that contact the ball at its equator and convert itsrotation into electrical signals representing orthogonal components ofmouse motion. These electrical signals are coupled to a computer, wheresoftware responds to the signals to change by a ΔX and a ΔY thedisplayed position of a pointer (cursor) in accordance with movement ofthe mouse. The user moves the mouse as necessary to get the displayedpointer to a desired location or position. Once the pointer on thescreen points at an object or location of interest, a button on themouse is activated with the fingers of the hand holding the mouse. Theactivation serves as an instruction to take some action, the nature ofwhich is defined by software in the computer.

In addition to mechanical types of pointing devices like a conventionalmouse, optical pointing devices have also been developed, such as thosedescribed in the incorporated patents and patent application. In oneform of an optical pointing device, rather than using a movingmechanical element like a ball in a conventional mouse, relativemovement between an imaging surface, such as a finger or a desktop, andphoto detectors within the optical pointing device, is optically sensedand converted into movement information.

It would be desirable to reduce the power typically consumed by anoptical pointing device. Limiting power consumption is particularlyimportant for portable electronic devices, such as portable computers,cellular telephones, personal digital assistants (PDAs), digitalcameras, portable game devices, pagers, portable music players (e.g.,MP3 players), and other similar devices that might incorporate anoptical pointing device.

Some optical motion sensors for optical pointing devices include alow-power mode that is automatically entered if no motion is detectedfor a period of time. In low power mode, power savings is achieved byturning off a light source of the optical pointing device. The lightsource is a major contributor to power consumption. The light source isturned back on if the optical motion sensor detects any movement, or thelight source is periodically turned back on to facilitate motiondetection. In some existing optical motion sensors, an undesirableswitch from the low power mode to a full power mode can be caused bynoise. If the optical motion sensor is on a border between pixels, theoptical motion sensor may report oscillations in motion as it attemptsto determine whether it is positioned just over or just under the nextpixel step threshold, which causes the optical motion sensor to leavethe low power mode. In addition, reasonably slow drift motions, such asthose caused by vibrations around an optical mouse, or those caused byplacing an optical mouse on a surface with a slight incline, can causean optical motion sensor to undesirably exit the low power mode.

In the low power mode in some optical motion sensors, images arecaptured, but at a significantly reduced rate compared to the rate atwhich images are captured in the full power mode. Some optical motionsensors provide 1500 “frame periods” per second. An image may or may notbe captured during a frame period. For example, in full power mode, animage may be captured during each frame period, resulting in 1500 imagesper second. In low power mode, an image may only be captured every 10 or12 frame periods, resulting in 125-150 images per second. In full powermode, the light source typically remains on for all frame periods, andis not turned off during a frame period or between frame periods. In lowpower mode, the light source is typically turned on only during frameperiods when images are captured, but remains on for the duration ofthose frame periods. Turning the light source on for only one frameperiod out of every 10 frame periods results in a reduction of the powerused for illumination of about 90 percent. It would be desirable toprovide further power savings in the low power mode, as well as areduction in power consumption in the full power mode.

Regardless of which mode an optical motion sensor is in, the lightsource remains on for the entire frame period when an image is captured.However, light is only needed for a small portion of a frame period. Aframe period includes three phases—an integration phase, an analog todigital conversion phase, and an image processing phase. Light is onlyneeded during a portion of the integration phase when an “electronicshutter” is open, allowing light to be collected. Power is unnecessarilyconsumed by leaving the light source on for the entire frame period.

It would be desirable to provide an optical screen pointing device withreduced power consumption.

SUMMARY OF THE INVENTION

One form of the present invention provides an apparatus for controllingthe position of a screen pointer for an electronic device having adisplay screen. The apparatus includes a light source for illuminatingan imaging surface, thereby generating reflected images. The apparatusincludes a motion transducer. A lens receives the reflected images anddirects the reflected images onto the motion transducer. The motiontransducer includes an electronic shutter for controlling the amount oftime that light is collected for image frames. The motion transducer isconfigured to generate digital representations of the reflected images.The motion transducer is configured to generate movement data based onthe digital representations of the reflected images. The movement datais indicative of relative motion between the imaging surface and themotion transducer. A controller coupled to the light source turns thelight source on only during the time that light is being collected foran image frame.

Another form of the present invention provides a method of controllingthe position of a screen pointer for an electronic device having adisplay screen. Light is directed from a light source onto an imagingsurface, thereby generating reflected images. The reflected images arefocusd onto an array of photo detectors. Output values of the photodetectors are digitized, thereby generating digital representations ofthe reflected images. At least one version of a first one of the digitalrepresentations is correlated with at least one version of a second oneof the digital representations to generate motion data indicative ofrelative motion between the imaging surface and the array of photodetectors. The light source is turned off during the digitizing andcorrelating steps. The position of the screen pointer is adjusted inaccordance with the motion data.

Another form of the present invention provides an apparatus forcontrolling the position of a screen pointer for an electronic devicehaving a display screen. The apparatus includes a light source forilluminating an imaging surface, thereby generating reflected images.The apparatus includes a motion transducer. A lens receives thereflected images and directs the reflected images onto the motiontransducer. The motion transducer includes an electronic shutter forcontrolling the amount of time that light is collected for image frames.The motion transducer is configured to generate digital representationsof the reflected images. The motion transducer is configured to generatemovement data based on the digital representations of the reflectedimages. The movement data is indicative of relative motion between theimaging surface and the motion transducer. A controller calculates atime average of the movement data. The controller is configured todetermine whether to switch the apparatus from a low power mode to afull power mode based on the calculated time average.

Another form of the present invention provides a method of switching anoptical screen pointing device from a low power mode to a full powermode. A first movement is detected with the optical screen pointingdevice. A first value representing an amount of the first movement iscalculated. An accumulated movement value representing an accumulationof previously detected movements is stored. The accumulated movementvalue is updated by adding the first value. The updated accumulatedmovement value is compared to a threshold value. It is determinedwhether to switch to the full power mode based on the comparison of theupdated accumulated movement value and the threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictographic side view illustrating the main components ofan optical, motion translation type screen pointer device according toone embodiment of the present invention.

FIG. 2 is an electrical block diagram illustrating major components ofone embodiment of a screen pointing device according to the presentinvention.

FIG. 3 is a timing diagram illustrating phases of a frame periodaccording to one embodiment of the present invention.

FIG. 4 is a flow diagram illustrating a process for reducing powerconsumption in an optical motion sensor according to one embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a parthereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

FIG. 1 shows a simplified representation of a side view of a motiondetection device 1 suitable for tracking the movement of a human finger7 pressed against a surface 5 of a transparent stud 3. A motiondetection device like that shown in FIG. 1 is described in detail in theabove-incorporated U.S. Pat. No. 6,057,540 (the '540 patent). Theoperation of motion detection device 1 is also summarized below.

When the tip 6 of finger 7 is pressed against surface 5, the ridges ofskin and any other micro texture features are visible in the plane ofsurface 5, just as if they were a part of surface 5. Lens 8 focuseslight from those features onto an array of photo detectors, which ispart of movement sensor 9. Movement sensor 9 automatically acquires andtracks any suitable image. When tracking an image, movement sensor 9produces incremental (X, Y) signals.

Lifting fingertip 6 away from surface 5 produces a loss of tracking.This condition is detected within motion detector 9, and in oneembodiment, the production of incremental (X, Y) signals ceases. Thishas the effect of leaving the position of the screen pointer unchangedat whatever location it currently occupies, and is exactly the same aswhen a user of a mouse removes his hand from the mouse. When fingertip 6is subsequently replaced on surface 5, motion detector 9 appreciatesthat an image has been acquired, and, in one embodiment, treats thatacquisition as though a reset has been performed. That is, until therehas been new motion subsequent to the new acquisition, the incrementalcoordinates (X, Y) will have the value (0, 0). This leaves the existingposition of the screen pointer undisturbed until such time as it isdeliberately moved by the motion of fingertip 6, and corresponds exactlyto a mouse user's placement of his hand back on the mouse without movingit.

An LED 2, which is an IR LED in one embodiment, emits light that isprojected by lens 4 onto a region 5 that is part of a work surface 6 tobe imaged for navigation. In one embodiment, motion sensor 9 is anintegrated circuit (IC) having an array of photo detectors, memory, andarithmetic circuits arranged to implement image correlation and trackingfunctions described herein and in the incorporated patents. An image ofthe illuminated region 6 is projected through an optical window (whichmay be transparent stud 3 itself) to a package (not shown) of integratedcircuit 9 and onto the array of photo detectors. Lens 8 aids in theprojection of the image onto the photo detectors.

One preferred optical navigation technique used by motion detectiondevice 1 involves optically detecting motion by directly imaging as anarray of pixels the various particular optical features visible atsurface 5, much as human vision is believed to do. IR light reflectedfrom a textured work surface pressed against surface 5 is focused onto asuitable array (e.g., 16×16 or 24×24) of photo detectors. The responsesof the individual photo detectors are digitized to a suitable resolutionand stored as a frame into corresponding locations within an array ofmemory.

FIG. 2 shows an electrical block diagram illustrating major componentsof motion detection device 1. Motion detection device 1 includes lightsource 2, lenses 4 and 8, and motion sensor 9. Motion sensor 9 includeslight sensitive current sources 148A-148C (collectively referred to ascurrent sources 148), electronic shutter 150 having a first plurality ofswitches 151A-151C (collectively referred to as switches 151) and asecond plurality of switches 153A-153C (collectively referred to asswitches 153). Motion sensor 9 also includes a plurality of sensecapacitors 154A-154C (collectively referred to as sense capacitors 154),multiplexer 156, amplifier 157, analog to digital (A/D) converter 158,correlator 160, system controller 162, shutter controller 164, and lightcontroller 166. In an alternative embodiment, only a single lens 8 isused, rather than two lenses 4 and 8.

The operation of motion sensor 9 is primarily controlled by systemcontroller 162, which is coupled to multiplexer 156, A/D converter 158,correlator 160, shutter controller 164, and light controller 166. Inoperation, according to one embodiment, light source 2 emits light thatis projected by lens 4 onto surface 6, which is a fingertip in one formof the invention. In an alternative embodiment, screen pointer device 1takes the form of an optical mouse, and surface 6 is a suitable surfacefor an optical mouse, such as a desktop. Light source 2 is controlled bysignals from light controller 166. Reflected light from surface 6 isdirected by lens 8 to light sensitive current sources 148. Currentsources 148 represent an array of photo detectors, and are also referredto as photo detectors 148. Photo detectors 148 each provide a currentthat varies in magnitude based upon the intensity of light incident onthe photo detectors 148.

Shutter switches 151 and 153 are controlled by a shutter signal fromshutter controller 164. Electronic shutter 150 is “open” when switches151 are open and switches 153 are closed, and electronic shutter 150 is“closed” when switches 153 are open. When shutter switches 151 are openand shutter switches 153 are closed (i.e., electronic shutter 150 isopen), charge accumulates on sense capacitors 154, creating a voltagethat is related to the intensity of light incident on photo detectors148. When shutter switches 153 are opened (i.e., electronic shutter 150is closed), no further charge accumulates or is lost from sensecapacitors 154. Multiplexer 156 connects each sense capacitor 154 inturn to amplifier 157 and A/D converter 158, to amplify and convert thevoltage from each sense capacitor 154 to a digital value. Sensecapacitors 154 are then discharged by closing switches 151 and 153.After discharging sense capacitors 154, switches 151 are opened so thatthe charging process can be repeated.

Based on the level of voltage from sense capacitors 154, A/D converter158 generates a digital value of a suitable resolution (e.g., one toeight bits) indicative of the level of voltage. The digital values forthe array of photo detectors 148 represent a digital image or digitalrepresentation of the portion of fingertip 6 positioned over surface 5of optical pointing device 1. The digital values are stored as a frameinto corresponding locations within an array of memory within correlator160. In one embodiment, each pixel in a frame corresponds to one of thephoto detectors 148.

The overall size of the array of photo detectors 148 is preferably largeenough to receive an image having several features (e.g., ridges in thewhorls of skin). In this way, images of such spatial features producetranslated patterns of pixel information as fingertip 6 moves. Thenumber of photo detectors 148 in the array and the frame rate at whichtheir contents are digitized and captured cooperate to influence howfast fingertip 6 can be moved across photo detectors 148 and still betracked. Tracking is accomplished by correlator 160 by comparing a newlycaptured sample frame with a previously captured reference frame toascertain the direction and amount of movement.

In one embodiment, the entire content of one of the frames is shifted bycorrelator 160 by a distance of one pixel successively in each of theeight directions allowed by a one pixel offset trial shift (one over,one over and one down, one down, one up, one up and one over, one overin the other direction, etc.). That adds up to eight trials. Also, sincethere might not have been any motion, a ninth trial “null shift” is alsoused. After each trial shift, those portions of the frames that overlapeach other are subtracted by correlator 160 on a pixel by pixel basis,and the resulting differences are preferably squared and then summed toform a measure of similarity (correlation) within that region ofoverlap. Larger trial shifts are possible, of course (e.g., two over andone down), but at some point the attendant complexity ruins theadvantage, and it is preferable to simply have a sufficiently high framerate with small trial shifts. The trial shift with the least difference(greatest correlation) can be taken as an indication of the motionbetween the two frames. That is, it provides raw movement informationthat may be scaled and or accumulated to provide display pointermovement information (ΔX and ΔY) of a convenient granularity and at asuitable rate of information exchange. Correlator 160 automaticallydetects when fingertip 6 has been removed from surface 5, by sensingthat all or a majority of the pixels in the image have becomeessentially uniform.

In addition to providing digital images to correlator 160, A/D converter158 also outputs digital image data to shutter controller 164. Shuttercontroller 164 helps to ensure that successive images have a similarexposure, and helps to prevent the digital values from becomingsaturated to one value. Controller 164 checks the values of digitalimage data and determines whether there are too many minimum values ortoo many maximum values. If there are too many minimum values,controller 164 increases the charge accumulation time of electronicshutter 150. If there are too many maximum values, controller 164decreases the charge accumulation time of electronic shutter 150.

In operation, images should be acquired at a rate sufficient thatsuccessive images differ in distance by no more that perhaps a quarterof the width of the array, or 4 pixels for a 16×16 array of photodetectors 148. Experiments show that a finger speed of 50 mm/sec is notunreasonable, which corresponds to a speed at the array of 800 pixelsper second. To meet a requirement of not moving more than four pixelsper cycle, a measurement rate of 200 samples per second is needed. Thisrate is quite practical, and it may be desirable to operate at severaltimes this rate.

FIG. 3 is a timing diagram illustrating phases of a frame period 300according to one embodiment of the present invention. A frame periodrepresents the time provided for capturing an entire frame of imagedata, and for analyzing the image data to determine movementinformation. Image data need not be captured every frame period. Forexample, when motion sensor 9 is in a low power mode, an image may onlybe captured every 10 or 12 frame periods. In one embodiment, when motionsensor 9 is in a full power mode, an image is captured every frameperiod.

Frame period 300 includes three phases—an integration phase 302, ananalog to digital (A/D) conversion phase 304, and an image processingphase 306. During integration phase 302, light is “collected” by photodetectors 148, and charge accumulates on sense capacitors 154 asdescribed above. During A/D conversion phase 304, the collected chargefrom sense capacitors 154 is converted into digital data by A/Dconverter 304 as described above. During image processing phase 306,correlator 160 processes the digital image data and generatesincremental movement signals (ΔX, ΔY) as described above.

In previous image sensors, in high power mode, the light source 2typically remained on for all frame periods, and in low power mode, thelight source 2 was typically turned on only during frame periods whenimages were captured. Regardless of which mode the sensor was in, foreach frame period that an image was captured, the light source remainedon for that entire frame period. However, light is only needed for asmall portion of frame period 300. Light is only needed during a portionof integration phase 302 when electronic shutter 150 is open, allowinglight to be collected. Power is unnecessarily consumed by leaving lightsource 2 on for an entire frame period 300.

In one embodiment of motion sensor 9, light source 2 is controlled byshutter signal 308 from shutter controller 164. Shutter signal 308 isshown in FIG. 3 below frame period 300. As shown in FIG. 2, shuttercontroller 164 is coupled to electronic shutter 150 and light controller166. When shutter signal 308 goes high, the high signal causes lightcontroller 166 to turn on light source 2. The high shutter signal 308also causes electronic shutter 150 to open, thereby allowing charge toaccumulate on sense capacitors 154. When shutter signal 308 goes low,the low signal causes light controller 166 to turn off light source 2,and causes electronic shutter 150 to close. Therefore, light source 2 isonly on during a portion of integration period 302, rather than duringthe entire frame period 300 as in previous motion sensors. As describedabove, the time that electronic shutter 150 is open is varied based onhow bright or dark the captured images are. Likewise, the time thatlight source 2 is on is varied to be on as long as the electronicshutter 150 is open. The time that electronic shutter 150 is open andlight source 2 is on is based on the length of time that shutter signal308 remains high. During the period of time in integration period 302prior to shutter signal 308 going high, sense capacitors 154 are resetor pre-charged to a desired starting value.

The time that electronic shutter 150 is open is typically substantiallyless than the time it takes to setup and process one image frame (i.e.,a frame period). In one embodiment, a frame period 300 is over 10,000clock cycles, whereas the electronic shutter 150 may only be open for 1or 2 clock cycles of a frame period 300. Thus, a 10,000 to 1 reductionin the amount of current used for illumination may be obtained for eachframe period 300 by only turning light source 2 on during the timeelectronic shutter 150 is open. Power is saved regardless of whethermotion sensor 9 is in a full power mode, or a low power mode.

As described above in the Background of the Invention section, in someexisting optical motion sensors, an undesirable switch from the lowpower mode to a full power mode can be caused by noise or reasonablyslow drift motions. In one form of the invention, motion sensor 9implements a process for avoiding this undesirable switch to full powermode, which includes time averaging motion values. FIG. 4 is a flowdiagram illustrating one embodiment of a process 400 implemented bymotion sensor 9 for reducing power consumption by avoiding such anundesirable switch to full power mode. To simplify the explanation,process 400 is described in the context of one-dimensional movement(i.e., movement in an X direction).

Process 400 begins with motion sensor 9 in a low power mode. In step402, a frame of image data is captured by motion sensor 9. In step 404,the captured frame is correlated with a previous frame by correlator160. Based on the correlation, correlator 160 determines ΔX in step 406,which represents the amount of the movement. In step 408, motion sensor9 updates a stored current accumulated ΔX value by adding the ΔXdetermined in step 406 to the stored current accumulated ΔX value.Motion sensor 9 then stores the updated value. In step 410, motionsensor 9 determines whether the current accumulated ΔX value (as updatedin step 408) is greater than a threshold value. In one embodiment, thethreshold value is 1, representing a one pixel movement per frame. Ifthe current accumulated ΔX value is not greater than the thresholdvalue, motion sensor 9 reduces the current accumulated ΔX by a decayfactor in step 412 and stores the reduced value. In one embodiment, thedecay factor is 0.5. In alternative embodiments, other decay factors areused. After reducing the current accumulated ΔX by the decay factor,motion sensor 9 remains in a low power mode, and jumps to step 402 torepeat the process. If the current accumulated ΔX value is greater thanthe threshold value in step 410, the ΔX motion data determined in step406 is reported in step 414. In step 416, motion sensor 9 enters a fullpower mode.

To further explain process 400, an example with movement values willdescribed. Assume that there has been no motion detected for a longperiod, and then a first movement occurs that is a one-half pixelmovement. Thus, in step 406, correlator 160 determines that ΔX=0.5. Instep 408, 0.5 is added to the current accumulated ΔX value (which isabout 0 since there has been no movement for a while). Thus, the updatedcurrent accumulated ΔX value is 0.5. Since the current accumulated ΔXvalue is not greater than 1 (step 410), motion sensor 9 reduces thecurrent accumulated ΔX to 0.25 (0.5× decay factor of 0.5) in step 412,and motion sensor 9 remains in a low power mode. Process 400 is thenrepeated, beginning at step 402.

Assuming that the next ΔX calculated in step 406 is also 0.5, thecurrent accumulated ΔX as updated in step 408 will be 0.75 (0.25+ thenew ΔX value of 0.5). Since the current accumulated ΔX value (0.75) isnot greater than 1 (step 410), motion sensor 9 reduces the currentaccumulated ΔX value to 0.375 (0.75× decay factor of 0.5) in step 412,and motion sensor 9 remains in a low power mode. Process 400 is againrepeated.

Assuming that the next ΔX calculated in step 406 is 1.0, the currentaccumulated ΔX as updated in step 408 will be 1.375 (0.375+ the new ΔXvalue of 1.0). Since the current accumulated ΔX value (1.375) is greaterthan 1 (step 410), motion sensor 9 reports the motion (step 414) andenters a full power mode (step 416).

Process 400 maintains the motion accuracy of motion sensor 9, buteffectively reduces the sensitivity of motion sensor 9 to go into a fullpower mode when small amounts of motion are reported. Power savings areobtained by remaining in low power mode in the presence of noise,vibrations, or slow drift motions that caused previous motion sensors toswitch to full power mode. By time averaging motion reports, motions farin the past are “forgotten”, and only current motions have a significanteffect in determining whether motion sensor 9 will enter full powermode. When motion stops, the current accumulated ΔX value continues todecay each frame period to zero. If motion reports are oscillating backand forth, for example, between +1 and −1 pixels, the time averagingfeature works to cancel out this type of noise.

Although the power savings techniques described herein are described inthe context of a finger pointing device, the techniques are alsoapplicable to an optical desktop mouse implementation.

It will be understood by a person of ordinary skill in the art thatfunctions performed by motion sensor 9 may be implemented in hardware,software, firmware, or any combination thereof. The implementation maybe via a microprocessor, programmable logic device, or state machine.Components of the present invention may reside in software on one ormore computer-readable mediums. The term computer-readable medium asused herein is defined to include any kind of memory, volatile ornon-volatile, such as floppy disks, hard disks, CD-ROMs, flash memory,read-only memory (ROM), and random access memory.

Although specific embodiments have been illustrated and described hereinfor purposes of description of the preferred embodiment, it will beappreciated by those of ordinary skill in the art that a wide variety ofalternate and/or equivalent implementations may be substituted for thespecific embodiments shown and described without departing from thescope of the present invention. Those with skill in the chemical,mechanical, electro-mechanical, electrical, and computer arts willreadily appreciate that the present invention may be implemented in avery wide variety of embodiments. This application is intended to coverany adaptations or variations of the preferred embodiments discussedherein. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method of switching an optical screen pointingdevice from a low power mode to a full power mode, the methodcomprising: detecting a first movement with the optical screen pointingdevice; calculating a first value representing an amount of the firstmovement; storing an accumulated movement value representing anaccumulation of previously detected movements; updating the accumulatedmovement value by adding the first value; comparing the updatedaccumulated movement value to a threshold value; determining whether toswitch to the full power mode based on the comparison of the updatedaccumulated movement value and the threshold value.
 2. The method ofclaim 1, and further comprising: switching to the full power mode if theupdated accumulated movement value is greater than the threshold value.3. The method of claim 1, wherein the threshold value is one pixel perframe. 4.The method of claim 1, and further comprising: reducing theupdated accumulated movement value by a decay factor.
 5. The method ofclaim 4, wherein the decay factor is 0.5.
 6. The method of claim 1,wherein the optical screen pointing device is configured to collect andprocess image data in each of a plurality of frame periods, each frameperiod including an integration phase during which light is collected,an analog to digital conversion phase during which collected light isconverted into digital values, and an image processing phase duringwhich image data is correlated with previous image data to determinemovement information.
 7. The method of claim 6, wherein the opticalscreen pointing device includes a light source, the method furthercomprising: turning the light source on only during the integrationphase of frame periods.